1. Field of the Invention
This invention relates to a wire electric discharge machining method for machining an entrance line to accurately intersect with a desired contour shape and an apparatus therefor.
2. Description of the Background Art
FIG. 4 is a block diagram showing a conventional wire electric discharge machining apparatus. In FIG. 4, reference numeral 1 denotes a wire electrode, 2 a workpiece to be machined in a programmed arbitrary shape by the wire electrode 1, 3 machining solution supply nozzles, 4 an arm for securing the upper machining solution supply nozzle 3, 5 an arm for securing the lower machining solution supply nozzle 3, 6 a pulley for altering the advancing direction of the wire electrode 1, 7 a machining solution tank, 8 a machining power source, 9 a table feed control unit for controlling the movement of a table for holding the workpiece 2, 10 a machining solution control unit, 11 a CPU for controlling the power source 8, the table feed control 9 and the machining solution control unit 10, and, finally, 12 a memory.
FIG. 5 shows a programmed machining route, in which a solid line indicates an already machined portion, and a broken line indicates a portion to be machined. Instructions for controlling the sequential movement of the workpiece 2 with respect to the electrode 1, often as a series of linear and/or curved loci, are stored in memory and retrieved to control the machining operation. FIG. 6 is an enlarged view of a portion A of FIG. 5.
In the operation of the conventional wire electric discharge machining apparatus, the wire electrode 1 and the workpiece 2 are opposed at a predetermined gap, and the wire electrode 1 is fed from a position above to a position below the workpiece 2. An electric discharge is produced in the gap between the wire electrode 1 and the workpiece 2 by electric energy supplied from the power source 8, through the machining solutions that are supplied from the upper and lower machining solution supply nozzles 3. The workpiece 2 is machined in a predetermined sequence to have an arbitrary shape, as shown in FIG. 5, based on instructions stored beforehand in the memory 12. The machining solutions are controlled to have desired machining solution parameters (e.g., pressure) by the machining solution control unit 10, and are supplied from the machining solution tank 7 to a location between the wire electrode 1 and the workpiece 2 from the upper and lower machining solution supply nozzles 3. As shown in FIG. 5, the shape programmed in advance is formed by controlling movement of the machining table, to which the workpiece 2 is secured, by the table feed control unit 9. The CPU 11 generally controls the power source 8, the table feed control unit 9 and the machining solution control unit 10.
With reference to FIG. 5, assume that the machining of a contoured hole having the shape of a square in a solid workpiece 2 is desired. First, a starting hole at point Pl in the portion of the workpiece 2 to be removed is formed by drilling or the like and the wire electrode 1 is threaded through the hole Pl for attachment between nozzles 3. Then, the wire electrode 1 is conveyed from a spool above the workpiece 2 for collection at a location below the workpiece. Machining solutions are supplied by machining solution control unit 10 from the machining solution tank 7 via the nozzles 3 to a location between the wire electrode 1 and the workpiece 2. Initially, the solution is supplied to the starting hole P1, the point where machining will begin. Then, electric energy is supplied from the power source 8 to start machining.
In accordance with a first machining instruction, the wire electrode is advanced from the starting hole along a predetermined machining entrance line (P1-P2). The entrance line intersects with, and in the example forms a 90.degree. angle with, the desired contour machining line between points P2 and P3. Then, in accordance with a second machining instruction, at point P2, the path of the electrode is changed and the workpiece 2 is continuously machined from the point P2 to a point P3. The machining proceeds to completion in response to a further series of instructions that returns the machining along line L1.
Since the line from the point Pl to the point P2 is only the entrance line, the point P2 actually is the starting point for machining the desired contour shape using the wire electric discharge machining apparatus. As the machining is continued according to the predetermined program, the hole is formed in a desired contour shape. As machining of the contour shape nears completion, the machining path returns along line L1 to the point P2. At point P2, the machining of the contour shape is completed, and the machining is considered finished, or the machining is further continued and a secondary machining is executed.
Clearly, the point P2 is both the contour shape starting point and the contour shape finishing point. Accordingly, the same point is machined twice. If the same point is machined twice under the same electric conditions, when the electric energy is supplied from the power source 8 to a point which already has been once machined, an electric discharge again occurs at the same point. However, the electric discharge gap due to the electric discharge is different from the gap that exists at a point which is machined only once. As a result, the point P2 is excessively machined and the workpiece 2 will have a defect at that point.
Since the conventional wire electric discharge machining apparatus is constructed and machined as described above, the point P2 on the machining line that is machined twice will be different from the point P3 which is machined only once. Specifically, the machined shape of the workpiece will be intruded at the point P2.
The present invention contemplates to solve the above-described problem and has as an object the provision of a wire electric discharge machining method and an apparatus therefor which can improve the machining accuracy of a workpiece without intrusion of a machining shape of the workpiece.